On Wed, 2014-11-05 at 11:06 -0600, Paul Clarke wrote:

Sorry it took me so long to get back to this...

On 10/07/2014 09:52 PM, Michael Ellerman wrote:

quoted

On Tue, 2014-07-10 at 19:13:24 UTC, Paul Clarke wrote:

quoted

This patch short-circuits the reset of the decrementer, exiting after
the decrementer reset, but before the housekeeping tasks if the only
need for the interrupt is simply to reset it.  After this patch,
the latency spike was measured at about 150 nanoseconds.

quoted

Thanks for the excellent changelog. But this patch makes me a bit nervous :)

Do you know where the latency is coming from? Is it primarily the irq work?

Yes, it is all under irq_enter (measured at ~10us) and irq_exit (~12us).

Hmm, OK. I actually meant irq_work_run().

AIUI irq_enter/exit() are just state tracking, they shouldn't be actually
running work.

How are you measuring it?

quoted

If so I'd prefer if we could move the short circuit into __timer_interrupt()
itself. That way we'd still have the trace points usable, and it would
hopefully result in less duplicated logic.

But irq_enter and irq_exit are called in timer_interrupt, before 
__timer_interrupt is called.  I don't see how that helps.  The time 
spent in __timer_interrupt is minuscule by comparison.

Right, it won't help if it's irq_enter() that is causing the delay. But I was
assuming it was irq_work_run().

Are you suggesting that irq_enter/exit be moved into __timer_interrupt 
as well?  (I'm not sure how that would impact the existing call to 
__timer_interrupt from tick_broadcast_ipi_handler?  And if there is no 
impact, what's the point of separating timer_interrupt and 
__timer_interrupt?)

The point is __timer_interrupt() is called from tick_broadcast_ipi_handler(),
which is called from smp_ipi_demux(), from icp_hv_ipi_action(), from
__do_irq(), which has already done irq_enter() (and will do irq_exit()).

cheers